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Flower-Mimicking Rust Fungi
Several fungal plant pathogens infect a host plant and alter its morphology in some fashion, but few alter the flowers of their hosts. Flower-mimicking rust fungi infect their host which results in the creation of pseudoflowers made by the host plant to aid the fungi in gamete dispersal for sexual reproduction. These rust fungi are parasites, requiring a living host plant in which to carry out their 5 life stages.
Species
Two species of rust fungi are known to infect and alter their plant host's flower morphology, resulting in the formation of structures on the plant that look similar to true flowers. These two species are Puccinia monoica and Uromyces pisi. Both species are in the same order of fungus, Pucciniales. [1]
Puccinia monoica
P. monoica is a crown rust that infects a range of species in the botanical genus Arabis within the family Brassicaceae, affecting 11 Brassicaceae species overall[2]. It also infects species outside of Arabis, notably Boechera stricta. It exists within a species complex.
Uromyces pisi
U. pisi infects a perennial ornamental plant, Euphorbia cyparissias[3] . It is also known to infect plants in the family Fabaceae[4]. It also exists within a species complex.
Characteristics
Pseudoflowers
Pathogenic rust fungi infect their host plant and induce the formation of pseudoflowers. These pseudoflowers are modified leaves that resemble flowers of another unrelated plant species.[2] The pseudoflowers differ from the host plant flowers in shape, size, color, scent, and nectar produced. These pseudoflowers are formed as yellow rosettes with embedded spermagonia and produce a sweet-smelling nectar to attract and reward insect pollinators[5]. The mimicry expressed here is similar to Dodsonian mimicry in which reproductive flowers are mimicked by another species. However, in Dodsonian mimicry, nectar is not produced by the impostor flower.
Scent Chemistry
The fragrance of the pseudoflowers produced by rust fungi is one of the main characteristics that attracts insect pollinators. The chemical compounds that form the fragrance of the fungal pseudoflowers differs between species. Mass spectrometry and gas chromatography analyses have allowed identification of different fragrances of these pseudoflowers. Curiously, some fragrances are found to be chemically similar to non-host flowers rather than to host flowers and host vegetation. P. monoica pseudoflowers are mainly composed of aromatic alcohols, aldehydes and esters[5]. In P. monoica, the fungal pseudoflower scent is dominated by aromatic compounds including phenylacetaldehyde and 2-phenylethyl alcohol. Comparatively, the scent of flowers of Arabis hosts is dominated by fatty-acid derived products. P. monoica's pseudoflowers are thought to be most similar to unspecialized Edelwiess flowers in terms of similarities in scent compounds.
In Uromyces pisi, the fungal pseudoflower scent is made of compounds including indole and jasmine lactone.[6] The scent of E. cyparissias pseudoflowers is known to vary based on geographic location, ranging from a floral-fruity smell to a spermy scent[6].
Life Cycle
Puccinia monoica life cycle
P. monoica has a macrocyclic, heteroecious life cycle. Macrocyclic refers to the fungus succesively completing 5 spore states in its life. Heteroecious refers to having two complementary mating types housed in separate organisms. The reproduction of a heteroecious species relies upon a dispersal mechanism in order for outcrossing between the mating types to occur. The life cycle of P. monoica is described in more detail here.
Uromyces pisi life cycle
U. pisi also has a macrocyclic, heteroecious life cycle[7], meaning it successively completes 5 spore stages in a complete life cycle. The five spores created are: Pycniospores (spermatia), Aeciospores, Urediniospores, Teliospores, and Basidiospores. Like P. monoica, outcrossing between mating types depends on a dispersal mechanism by an insect pollinator. U. pisi alternates between two plant hosts to proceed through its life cycle. E. cyparissias hosts the spermatia and aeciospores, while a species in the family Fabaceae is host to the other spore types[8].
Ecology
Pseudoflower-insect interactions
Rust fungi reproduction depends on insect visitation, analogous to pollination in flowers. Insects transfer spermatia (gametes) between different mating types of infected plants. The presence of the pseudoflower in plant communities may have consequences other than enhance the reproduction of the fungus, studies in Puccia monoica demonstrated that insect visitation occurs more often when plants present a mixture of both, pseudoflowers and natural occurring flowers, rather than when they occur alone in the plant. Combination of real flowers and pseudoflowers in plants increase the floral display through and apparent higher density, it also produces complementary attraction cues and complementary pollinators rewards, thus it has been proposed that they play a role in the pollination dynamics between insect and plants [9]. In U. pisi, insect pollinator visits are also necessary in order for two mating types of U. pisi to interact and result in fertilization[3].
See also
References
- Pfunder, Monika, and Bitty A. Roy. "Fungal pseudoflowers can influence the fecundity of insect-pollinated flowers on Euphorbia cyparissias." Botanica Helvetica 116 (2006): 149 - 158.
- Ngugi, H. K., and Harald Scherm. "Mimicry in plant-parasitic fungi." FEMS Microbiology Letters 257 (2006): 171 - 176.
- Cano, L.M., et al. "Major Transcriptome Reprogramming Underlies Floral Mimicry Induced by the Rust Fungus Puccinia monoica in Boechera stricta." Plos ONE 8(9) (2013): 1 - 16.
- ^ http://website.nbm-mnb.ca/mycologywebpages/NaturalHistoryOfFungi/Pucciniales.html
- ^ a b Roy, B. A. "Floral Mimicry by a Plant Pathogen." Nature 362 (1993): 56 - 58.
- ^ a b Pfunder, Monika, and Bitty A. Roy. "Pollinator-mediated interactions between a pathogenic fungus, Uromyces pisi (Pucciniaceae), and its host plant, Euphorbia cyparissias (Euphorbiaceae)." American Journal of Botany 87 (2000): 48 - 55.
- ^ Pfunder, Monika; Schürch, Stéphanie; Roy, Barbara A. "Sequence variation and geographic distribution of pseudoflower-forming rust fungi (Uromyces pisi s. lat.) on Euphorbia cyparissias". Mycological Research. 105 (1): 57–66. doi:10.1017/s0953756200003208.
- ^ a b Raguso, Robert A., and Barbara A. Roy. "‘Floral’scent production by Puccinia rust fungi that mimic flowers." Molecular Ecology 7.9 (1998): 1127-1136.
- ^ a b Kaiser, Roman (2006-02-10). "Flowers and Fungi Use Scents to Mimic Each Other". Science. 311 (5762): 806–807. doi:10.1126/science.1119499. ISSN 0036-8075. PMID 16469916.
- ^ Barilli E., Sillero J.C., Prats E., Rubiales D. "Resistance to rusts (Uromyces pisi and U. viciae-fabae) in pea." Czech J. Genet. Plant Breed 50 (2014): 135–143.
- ^ GAUMANN, E. Die Rostpilze Mitteleuropas. Beitrage zur Kryp-togamenflora der Schweiz Band 12 (1959): 1–1407.
- ^ Roy, B. A. (1994-03-01). "The Effects of Pathogen-Induced Pseudoflowers and Buttercups on Each Other's Insect Visitation". Ecology. 75 (2): 352–358. doi:10.2307/1939539. ISSN 1939-9170.